Many substances that you encounter in your daily life have different properties. Substances have either acidic, basic or neutral properties.
Acids: Acids are sour to the taste and include substances like curd, vinegar, lemon, orange juice, etc. These contain natural acids. These substances are acidic in nature.
Bases: Bases are bitter to the taste and slippery to the touch and include substances like baking soda milk of magnesia, etc.
Indicators: There are special substances that are used to test whether something is acidic or basic. These substances are called indicators. Indicators change their colour when added to an acidic or basic solution. Some natural indicators are litmus, turmeric and china rose petals.
Indicators
Litmus is a common natural indicator extracted from lichens.
Litmus has a purple colour in a neutral medium, red colour in an acidic medium, and blue in a basic medium.
When a drop of freshly prepared lemon juice is put on red litmus, it remains red, indicating the acidic nature of lemon juice.
If blue litmus is used instead, then it turns red, again indicating the acidic nature of lemon juice.
A solution of litmus turns red if it is acidic, and blue if it is basic.
S. No | Name of acid /base | Effect on litmus paper | Effect on turmeric paper |
1 | Hydrochloric acid | Blue litmus turns red | Remains yellow |
2 | Sulphuric acid | Blue litmus turns red | Remains yellow |
3 | Sodium hydroxide | Red litmus turns blue | Turns reddish brown |
4 | Potassium hydroxide | Red litmus turns blue | Turns reddish brown |
Turmeric changes colour to indicate acidic and basic substances.
Turmeric paper changes colour in a basic medium.
In an acidic medium, it remains yellow, while in a basic medium, it turns reddish-brown.
When a drop of soap solution is put on turmeric paper, it turns red. This indicates the basic nature of soap solution.
S. No | Test solution | Effect on turmeric paper |
1 | Lemon juice | Yellow |
2 | Orange juice | Yellow |
3 | Lime water | Reddish-brown |
4 | Milk of magnesia | Reddish-brown |
China rose is also a natural indicator extracted from China rose petals. A solution of china rose turns green in a basic solution and bright pink or magenta in an acidic solution.
S.No. | Name of the acid | Effect on China rose |
1 | Hydrochloric acid | Dark red |
2 | Sulphuric acid | Dark red |
3 | Sodium hydroxide | Green |
4 | Potassium hydroxide | Green |
Acid rain is rain that contains a lot of acid. This acid is created when pollutants in the air, such as carbon dioxide, sulphur dioxide and nitrogen dioxide, dissolve in rainwater, and form acids like carbonic acid, sulphuric acid and nitric acid
This rain causes a lot of damage to buildings, plants and animals. This is one important reason why we must avoid polluting our atmosphere.
Neutralisation is the reaction between an acid and a base to form salt and water. During neutralisation, salt and water are formed, along with the release of some heat.
Eg: HCl + NaOH------------>NaCl + H2O + Heat
Eg: H2SO4 + 2NaOH--------------->Na2SO4 + 2H2O + Heat
The neutralisation reaction between an acid and a base can be identified by an indicator.
For example, for the reaction between hydrochloric acid and sodium hydroxide, phenolphthalein is added as an indicator. Phenolphthalein is pink in sodium hydroxide. When hydrochloric acid is added to it, the colour slowly fades to pale pink and then colourless. This change in colour indicates neutralisation.
The stomach contains hydrochloric acid, which helps in the digestion of food. However, when this acid is produced in excess, it causes indigestion, which is painful. An antacid such as milk of magnesia neutralises the excessive acid in the stomach and provides relief from the pain due to indigestion.
Ant bite contains formic acid. When an ant bites, it injects this formic acid into the skin. This causes pain. It can be neutralised by rubbing the ant bite with moist baking soda, which contains sodium hydrogen carbonate (NaHCO3), or with a solution of calamine, which contains zinc carbonate (ZnCO3).
Excessive use of fertilisers makes the soil acidic. As a result, plants cannot grow properly, and the yield decreases. Adding a base like quick lime, (calcium oxide) or slaked lime (calcium hydroxide) neutralises the soil and makes it suitable for plants.
Similarly, when soil is basic in nature, organic matter is added to release acid and make it neutral, and thus, suitable for plants.
Factory waste contains acids. This waste must be treated with bases for neutralisation before it is released into a water source. Otherwise, it can cause damage to living organisms in the water source.
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